Here's an update from my friend's group... I guess he'll be sending me these as they get written...
"Growth comes from food, not light. Here is a nice report/study that helps explain what is really going on:
"Effect of Natural Zooplankton Feeding on the Tissue and Skeletal Growth of the Scleractinian Coral Stylophora Pistillata. Publication: Coral Reefs, 2003"
"Coral nutrition has been of interest to marine ecologists because of the high biomass concentration of reef corals, compared to the paucity of nutrients in the tropical waters where they live. Scleractinian corals [such as Stylophora] are heterotrophs, able to utilize a variety of food sources such as sediment, dissolved and particulate organic matter, bacteria, and zooplankton. They are also photo-autotrophs [feed from the sun] because they are symbiotic with unicellular dinoflagellates [algae in the coral's skin] commonly called zooxanthellae. Such multiple modes of nutrition seem to be an evolved mechanism for coping with oligotrophic [low nutrient] tropical waters."
"Photo-autotrophy [feeding from the sun] has been well documented, and it is clear that corals obtain large amounts of photosynthetically fixed carbon [energy] from their symbiotic algae through translocation. It has been suggested that these photosynthetic products are mostly used for respiration [to stay alive], with only a small proportion being used for growth and reproduction, because such products [from the sun] are deficient in essential nutrients such as nitrogen and phosphorus [which are needed for growth]."
"The main questions of [our] work is as follows: (1) Does feeding increase the amount of coral tissue, and at what rate? (2) Since zooxanthellae can use the catabolic end products of [the coral's] metabolism, does feeding also affect the [zooxanthellae] by changing their amount of chlorophyll or their cell density? (3) Does feeding change the coral's calcification [growth] rate? (4) How do light levels and food supply interact to affect calcification and tissue growth rates?"
"Laboratory experiments were designed to estimate the ingestion rates of the scleractinian coral Stylophora pistillata under varying prey [food] concentrations, and feeding regimes, and to assess the effect of feeding on the tissue and skeletal growth. Six sets of corals were incubated under two light and three feeding levels (none, fed twice, and fed six times per week), using freshly collected zooplankton. Results showed that the number of prey [food] ingested was proportional to the prey density [amount of food in the water], and that no saturation of feeding capability was reached. Capture rates varied between 0.5 and 8 prey items per 200 polyps per hour. Corals starved for several days ingested more plankton than did fed corals. Fed colonies exhibited significantly higher levels of protein, chlorophyll a, and chlorophyll c2, per unit surface area than starved colonies did. Feeding had a strong effect on tissue growth, increasing it by two to eight times. Calcification [growth] rates were also 30% higher in fed than in starved corals. Even moderate levels of feeding enhanced both tissue and skeletal growth, although the processes involved in this enhancement remain to be determined."
"In January 2000, 60 randomly selected micro-colonies [of stylophora] were divided into four aquaria containing 15 corals each. Growth of each [frag] was monitored during 2 weeks before the start of the experiment (control period). Two groups were then fed 6 days per week, for 6 weeks, with freshly collected zooplankton, while the other two groups remained starved. Each fed and starved group was maintained at two light intensities. The tank positions were changed regularly to avoid any uneven light effect. Corals within each aquarium were also rotated to avoid any position difference (flow, light) effects within the aquaria."
"In July 2000, 120 microcolonies were divided into six aquaria containing 20 corals each. Growth rates were also followed during 2 weeks before the start of the experiment. Two groups were then fed six times a week for 6 weeks with zooplankton (highly fed group). Two other groups were fed only twice a week (slightly fed group). The last two groups remained starved (starved group). Each group was maintained at the same light intensities, and treated as above."
"Plankton were collected from [the ocean] every morning and brought back immediately to the laboratory. [...] After feeding, the tanks were entirely emptied and refilled with new seawater."
"When presented with food, starved corals captured plankton at significantly higher rates than slightly and highly fed groups; there was no significant difference between the slightly and the highly fed groups."
"For both experiments, the amount of [coral tissue] per unit surface area was significantly higher in fed than in starved corals, and at both light levels, by the end of the experiment; fed corals contained 30 to 49 percent more [coral tissue] per square centimeter than starved corals. There was, however, no significant difference between coral groups fed twice and six times per week. There was a strong effect of feeding and light on these [tissue growth] rates. Feeding and light significantly increased the amount of tissue produced over a given area of skeleton."
"Skeletal growth rates, calculated for the entire incubation time, were significantly higher for fed corals, at both light levels, for the January experiment and under low light during the July experiment. [...] Fed corals experienced 50 to 73 percent more growth in January under high and low light, respectively, and 46 percent more growth in July under low light."
"Feeding is one of the least-understood aspects of coral biology, possibly because significant quantities of identifiable prey [food] items are difficult to find in [coral stomach] samples without rapid preservation, and because the sampling process is laborious. [...] Stylophora pistillata, despite its relatively small polyp size, captures and ingests zooplankton readily, mainly copepods, in these experiments."
"The number of [pods] ingested was proportional to prey density [food in the water]."
"Feeding status had an important effect on these rates; corals maintained under starvation for several days ingested more plankton than did well fed corals. However, capture rates were not significantly different between slightly and highly fed groups. It seems unlikely, that in the case of the slightly fed group in particular, a saturation in feeding response would be responsible for this difference, given the 2-day interval between feeding bouts. [A previous study] suggested that the time spent digesting prey limits the amount of food consumed, since prey are not captured while there is food in the gut [thus, feeding corals all at one time wastes the majority of the food]"
"Results obtained in this study show that feeding had a strong effect on tissue growth, increasing it two and eight times in July and January, respectively. These results indicate that, even during optimum light conditions, photo-autotrophy [feeding from the sun] cannot satisfy the maximal daily energy and nutrient requirements of corals for both maintenance and growth."
"This study also strengthens the previous results showing higher calcification [growth] rates in fed than in starved corals. The enhancement of both tissue and skeletal growth with feeding suggests that the coral Stylophora pistillata allocates a high proportion of the energy brought by food to growth, when food is available. [...] In this study, fed corals grown under low light were able to maintain their growth rates, while the growth rates of starved corals dramatically decreased during the incubation. The highest growth was, however, observed when corals utilized both heterotrophic [food] and photo-autotrophic [light] nutritional sources."
"Whatever limits calcification [growth] rates, feeding removes some of that limitation."
Attached below is Figure 5 from the study
(yes I just learned that poop is coral food)
